(1) Field of the Invention
The present invention relates to an improvement on an oxygen sensor or a so-called waterproof type oxygen sensor, comprising a sensor element having electrodes on inner and outer surfaces, respectively, which are to contact with air and exhaust gases, respectively, a housing for containing the sensor element, a gas-tightly tight sealed section provided in the housing for separating the exhaust gases from the air, and a water-repelling communicating section for communicating the air in the gas-tightly sealed section with surrounding air.
(2) Related Art Statement
Waterproof type oxygen sensors have conventionally widely been known as oxygen concentration detectors for exhaust gases in automobiles. FIGS. 2a and 2b and FIG. 3 show examples of the waterproof type oxygen sensors to which the present invention is applicable. In these figures, the same reference numerals denote the same or similar parts.
Since the oxygen sensor of this type uses air as a reference oxygen atmosphere, it is constructed to isolate air as the reference oxygen atmosphere from gases to be measured. As a way to separate air from the gas to be measured, in the case of a planar sensor element 12 as shown in FIG. 2a, a talc 8 is filled between a metallic cap 10 and a sensor element 12. In the case of an oxygen sensor as shown in FIG. 3, a metal sealing structure using a metal packing 11 is employed.
As shown, the inner electrode is separated from the gases to be measured, and an air introduction path, which rejects invasion of water is provided to introduce surrounding air into the inner electrode.
As the water-repelling air introduction path, gaps among twisted filaments of a lead wire are utilized as the air introduction path, and an opened end of the lead wire to which no water is applied is utilized as an air introduction opening. Alternatively, as shown in a cross-sectional detailed view at portion A of FIG. 2a and FIG. 2b, a ring 16 made of a water-repelling material is provided between the metallic cap 10 and a metal fixing member 13 inside a metallic boot 9, and the ring 16 is provided with small gaps 14 to repel water and pass air only. Further, as shown in FIG. 3, it is proposed that a water-repelling porous member 19 is provided at an air inlet opening 18.
However, when such oxygen sensors are used for a long time period under high load-running conditions, electromotive forces drop.